In a vehicular control unit, signals inside the control unit are used for diagnosing operation and functions of the control unit. However, such signals are difficult to take out of the control unit because the control unit is generally installed in a hard-to-reach location. Interior parts, surrounding devices, and brackets and stays that hold the control unit need to be removed to reach the control unit in many vehicles.
Furthermore, control signals or information signals are dramatically increased according to an increase in demand for improvements in vehicles in terms of intelligence and safety. As a result, harness assembly becomes complicated and a diameter of a bundle of the harnesses becomes large. The removal of the control unit requires a large number of man-hours due to the complicated harness assembly.
A diagnostic system or a development support system, such as a failure diagnosis device proposed in JP-A-2002-301997, is directly connected to an internal network of a vehicle. An additional signal line or harness is not required for transferring signals or data between the diagnostic system and the control unit.
Communication between the diagnostic system or the development support system, that is, an external system, and the control unit is often interrupted due to busy traffic in a communication line. Moreover, it may be interrupted due to transmission of signals or data related to driving control of the vehicle depending on priority or urgency. In such a condition, a difference between a sensor output received by the control unit and an actual sensor output is not precisely determined. Furthermore, internal communication cannot be examined in real time even when detailed investigation of operation and function of the control device is required.